A spark plug for an internal combustion engine, such as vehicle engine, generally includes a rod-shaped center electrode, a substantially cylindrical insulator disposed around an outer circumference of the center electrode, a substantially cylindrical metal shell disposed around an outer circumference of the insulator and a ground electrode joined to a front end of the metal shell so as to define a spark discharge gap between a front end portion of the center electrode and a distal end portion of the ground electrode.
There has recently been a demand to reduce the size and diameter of the spark plug for the purpose of improvements in engine layout flexibility. For example, it is conceivable to reduce the diameter and thickness of the metal shell for size/diameter reduction of the spark plug.
It is common practice to fix the metal shell to the insulator by inserting the insulator through the metal shell, with the center electrode fitted in a front end side of the insulator, and radially inwardly crimping a rear end portion (also referred to as “crimp portion”) of the metal shell onto the insulator. The crimp portion of the metal shell is made relatively thin so as to be crimped onto the insulator. When the metal shell is made small in diameter and thickness, such a thin crimp portion deteriorates in strength and cannot be crimped with a sufficient force to ensure air tightness.
As a solution to the above problem, Japanese Laid-Open Patent Publication No. 2003-257584 (abbreviated as “JP 2003-257584 A”) discloses a spark plug that has a metal shell formed of a steel material and including a crimp portion with a reduced cross-sectional area, in which the steel material of the metal shell has a carbon content increased depending on the cross-sectional area of the crimp portion so as to impart strength to the crimp portion such that, even when the metal shell is made small in diameter, the crimp portion can be fixed to an insulator with a sufficient crimping force to improve air tightness and vibration resistance (see paragraph [0009] of JP 2003-257584 A).